11.3 – Optical Phenomena in Nature

Rainbows The sun must be behind you to see a rainbow, and it must reflect off of water droplets in the air The colours you see are due to a combination of reflection, refraction, and dispersion through these water droplets

Rainbows A rainbow forms when light enters a water droplet and refracts, reflects off the inner surface of the droplet, and then refracts again when leaving the droplet, causing dispersion

Sundogs Sundogs appear as bright spots on both sides of the sun They appear when ice crystals in the atmosphere refract sunlight (as opposed to water droplets for rainbows)

The Illusion of Apparent Depth An image can be formed by either reflection of light (as in a mirror) or refraction of light You can use ray diagrams to show where the refracted image is located Apparent depth is an optical effect in which the image of an object appears closer than the object itself Often occurs when you are looking from the air into water

The Illusion of Apparent Depth Light rays from the object travel to your eyes, but the rays have refracted at the surface of the water The image of the box is located by tracing the refracted rays backwards until they meet (just like for reflection!) Notice that the box on the bottom of the pool looks like it is higher than it actually is – in reality, the bottom of the pool is deeper than it appears to be

Shimmering and Mirages Shimmering and mirages are caused by refraction of light in unevenly heated air When light travels through air at different temperatures, it refracts because hot air is less dense than cold air Because there is no distinct boundary between sections of air at different temperatures, the light does not bend at one specific point – it travels along a curved path Also, since air is usually moving, the direction and the amount of bending are constantly changing

Shimmering You can see this in the air above any hot surface – the objects look wavy

Mirages Occurs on a much larger scale than shimmering Most commonly seen in very hot deserts or on highways – the sand or paved surface becomes extremely hot after being in sunlight for several hours The hot ground heats the air just above it, making the lower layer of air much hotter than the higher air When sunlight reaches the hot air near the ground, the sunlight is refracted upwards

Mirages An object that appears to be on the ground but is not really there is called a mirage The solid, curved line below shows the path of light from the sky; the dashed line shows how your brain interprets the scene